Core balance earth leakage protective systems

ABSTRACT

A core balance earth leakage protective system including a magnetic core; first and second sensing windings on the core; an electronic amplifier, the input to the amplifier being connected to the first sensing winding on the core and the output from the amplifier being electro-magnetically coupled with the core; a polarized magnetic relay including an actuating coil which is connected to the second sensing winding on the core, the relay being operative to cause isolation of load windings associated with the core when the current through the relay coil exceeds a predetermined value.

United States Patent Cohen et al.

[ CORE BALANCE EARTH LEAKAGE PROTECTIVE SYSTEMS Inventors: Vivian Cohen,Johannesburg; Sidney Charles Slocombe, Brakpan, both of South Africa[21] Appl. No.: 294,564

[ 1 June 26, 1973 3,558,980 1/1971 Florance et al 317/18 D 3,566,1892/1971 Wilson et al 317/18 D 3,668,470 6/1972 Ambler et al 317/18 DPrimary Examiner.lames D. Trammell Attorney-Ostrolenk, Faber, Gerb &Soffen [57] ABSTRACT A core balance earth leakage protective systemincluding a magnetic core; first and second sensing windings on thecore; an electronic amplifier, the input to the amplifier beingconnected to the first sensing winding 0n the core and the output fromthe amplifier being [52] U.S. Cl. 317/18 D, 317/27 R lectr -magneticallycoupled with the core; a p larized [51 Int. Cl. 1102b 3/28 m gn i relayincluding an actuating coil which is con- [58] Field of Search 317/18 D,27 R n e t he second sensing winding on the core, the

relay being operative to cause isolation of load wind- [56] ReferencesCited ings associated with the core when the current through UNITEDSTATES PATENTS the relay coil exceeds a predetermined value.

3,548,259 12/1970 McDonald 317/18 D 10 Claims, 1 Drawing Figure 2 J l JI E 2 l 12 l r [4 15 11 I :I 6 K PATENTEUJUNZB Ian m w L Tw i w .v

CORE BALANCE EARTH LEAKAGE PROTECTIVE SYSTEMS This invention relates tocore balance earth leakage protective systems.

At the present time, earth leakage protective devices generally fallinto either of two basic classes, namely:

a. the transistorized electronic type, or

b. the polarized relay type.

Irrespective of type, all earth leakage protective devices normallyinclude three fundamental circuit components, namely:

i. a monitor comprising a toroidal magnetic core, usually of the lowloss-high permeability type, and a sensing winding on the core totransmit a monitored earth fault signal to an amplifier, and

ii. an amplifier comprising either a transistorized electronic amplifieror a polarized magnetic relay, depending on the basic class to which thedevice belongs, and

iii. a tripping device which normally comprises a circuit breaker oron-load switch which is operated indirectly via an associated shunt tripmechanism or, alternatively, directly by the polarized magnetic relay.

It is well known that the transistorized electronic type of earthleakage device, as well as the polarized relay type of device, hasadvantages but also disadvantages. It is generally accepted that thetransistorized electronic type of device has the advantage of long termreliability with accurate stability and repeatability over wide rangesof ambient temperature and supply voltage, that it responds to lowimpedance neutral faults, is stable against mechanical shock orvibration, is unaffected by lightning surges and by short circuitcurrent up to the rupturing limit of associated circuit breakers.

However, transistorized electronic type devices suffer from thedisadvantage that a single phase unit will not operate under conditionsof broken neutral and that a three phase unit will not operate if theshunt trip supply is lost.

A polarized relay type of device, on the other hand, has the advantagethat it can still operate with a broken neutral and that a three phasedevice is unaffected by the loss of one supply phase. However, this typeof device suffers from the disadvantages that a neutral fault on certainsystems results in a drastic reduction in sensitivity even to the pointof non-operation, that long term stability cannot be guaranteed in thesame way as the transistorized electronic type of device and that it ismore susceptible to mechanical shock than the transistorized type.

It will be appreciated from the foregoing that neither of the two basictypes of earth leakage protective devices is entirely satisfactory tomeet all possible operating conditions and requirements. In recent yearsthe importance of earth leakage protection and the moral obligation ofprotecting human life has been receiving increasing recognition. In aneffort to avoid the disadvantages of each of the two basic types ofdevices when used on its own, it has been suggested to provide the twotypes of devices together in parallel connection to serve as back-upprotection one for the other. Improved results have been achieved but asfar as applicants are aware, the results achieved have not been assatisfactory as might be desired.

It is accordingly an object of the present invention to provide a novelearth leakage protection system which applicants believe will giveimproved protection.

According to the invention a core balance earth leakage protectivesystem includes a magnetic core; first and second sensing windings onthe core; an electronic amplifier, the input to the amplifier beingconnected to the first sensing winding on the core and the output fromthe amplifier being electro-magnetically coupled with the core; apolarized magnetic relay including an actuating coil which is connectedto the second sensing winding on the core, the relay being adapted tocause isolation of load windings associated with the core when thecurrent flow through the relay coil exceeds a predetermined value.

A plurality of single or multi-turn load windings may be provided on thecore. Alternatively, the core may be adapted for a plurality of loadconductors to be passed through or round the core to constitute aplurality of single or multi-turn load windings. This aspect will beclear to a man skilled in the art.

When an earth fault occurs, the amplifier is energized by a signalderived from the first sensing winding and the polarized relay isenergized from the second sensing winding.

With the arrangement according to the invention, the amplifier and thepolarized relay are not merely connected together in parallel. Also, theamplifier is not adapted to be connected to its own tripping device, bututilizes the polarized relay to cause isolation of the load windings,the output of the amplifier being electromagnetically coupled to therelay coil through the core and the second sensing winding so that whenthe amplifier is energized by an earth fault signal derived from thefirst sensing winding, the amplifier acts to energize the relay coil andcause isolation of the load windings if the predetermined current flowin the relay coil is exceeded. It will be appreciated that the secondsensing winding acts as a coupling winding between the amplifier and therelay.

Since the relay coil is connected to the second sensing winding on thecore, the coil can be independently energized by an earth fault signalderived directly from the second sensing winding. The arrangement issuch that when an earth fault occurs, the energization of the relay coilby a signal derived from the second sensing winding due to the earthfault alone, reinforces the energization of the relay coil by a signalemanating from the amplifier. This normally increases the sensitivity ofthe system and the overallgain that can be achieved.

In a preferred embodiment of the invention, the amplifier part of thesystem is adapted to be more sensitive to earth faults than thepolarized relay part, so that normally isolation of the load windingswould be caused by energization of the relay coil from the amplifierbefore sufficient energization of the relay coil would be obtained froma signal derived from the second sensing winding as a result of an earthfault alone. In other words, the advantages of an electronic amplifierprotective device can normally be relied upon, the polarized relay partbeing relied upon to act independently in the event of the amplifierpart becoming inoperative due to loss of supply or any other cause.

The amplifier is preferably of the transistorized type and may be ofconventional design. The amplifier may derive its power supply from theload conductors which is protected by the earth leakage protectivesystem.

The output from the amplifier may be connected to a separate couplingwinding on the core in order electro-magnetically to couple the outputwith the core.

Preferably, the output from the amplifier is connected to the firstsensing winding or part thereof, such as by impedance coupling.

Preferably also, the winding on the core to which the output from theamplifier is connected, forms part of a regenerative feedback circuit.

It will be appreciated that a transformer coupling is provided betweenthe amplifier and the relay coil through the core. This has severaladvantages. For example, a small relay coil having a low impedance maybe used, the transformer coupling providing impedance matching. Thisfacilitates miniaturization. Further, the level of an actuating signalapplied to the relay coil can be enhanced, particularly when aregenerative feedback system is used. This increases the sensitivity ofthe system and the overall gain that can be achieved.

A regenerative feedback system coupled to the core has the furtheradvantage that magnetic set in the core can be overcome or at leastminimized.

With a system according to the invention a sensitive system withexcellent long term reliability and stability can be obtained withoutsupersensitive individual circuit components being required.

For a clear understanding of the invention, a preferred embodiment ofthe invention will now be .described by way of example with reference tothe accompanying diagrammatic circuit diagram.

Load 1 is connected by load conductors 2, 3, to the line and neutralterminals 4, 5 respectively of a 3-phase alternating current powersupply through circuit breaker 6. Load conductors 2, 3 are connected toor constitute load windings 7, 8 respectively on a toroidal magneticcore 9.

Transistorized electronic amplifier 10 of conventional design isconnected to power supply circuit 11, I

which in turn, is connected between load conductors 2, 3. The inputterminals 12, 13 of amplifier 10 are connected to a first sensingwinding 14 on core 9 and the output terminals 13, 15 of amplifier 10 areconnected across a portion of first sensing winding 14, the arrangementbeing such that an impedance coupled regenerative feedback circuit isobtained. This aspect will be clear to a man skilled in the art.

Polarized relay 16 includes an actuating coil (not shown) which isconnected to second sensing winding 17 on magnetic core 9 and anarmature (not shown) which is adapted directly or indirectly to causetripping of circuit breaker 6 when current which flows through the relaycoil and which is derived from the second sensing winding 17, exceeds apredetermined value.

The amplifier part of the system is more sensitive to monitored earthfault signals derived from an unbalance in the algebraic sum of thenegative phase sequence components of the currents flowing in loadconductors 2 and 3 due to earth fault current flow, than the polarizedrelay part. In other words, the earth fault current required to causetripping of circuit breaker 6 under the influence of the amplifier partof the system is smaller than the earth fault current that is necessaryto cause tripping of the circuit breaker 6 under the influence of thepolarized relay part on its own.

In the event of an earth fault occurring with the amplifier part infully operative condition, the first sensing winding 14 applies amonitored earth fault signal to the input of amplifier 10, thereby toproduce an amplified output which is transferred electro-magneticallythrough first sensing winding 14 or part thereof and core 9 to secondsensing winding 17 and which is applied from second sensing winding 17to the relay coil to energize the latter. Additionally, second sensingwinding 17, additively applies to the relay coil an earth fault signalwhich it has monitored independently. When the resultant current flowthrough the relay coil exceeds the predetermined level, the relay causescircuit breaker 6 to trip and isolate load 1 from the power supply.

In the event of the amplifier part of the protective system becominginoperative, such as may be caused by a broken neutral which wouldinterrupt the power supply to the amplifier 10, the polarized relay partacts independently on its own to cause tripping of the circuit breaker 6when the current which flows through the relay coil and which is derivedfrom an earth fault signal monitored by second sensing winding 17,exceeds the predetermined value.

It will be appreciated that many variations in detail are possiblewithout departing from the scope of the appended claims. Any suitablepolarized relay may be used, such as that disclosed in our US. Pat. No.3,475,708 dated Oct. 28, 1969.

[n the case where amplifier It) employs an autotransformer type ofoutput coupling, terminals 12, 13 may also constitute the outputterminals of amplifier 10, the amplifier output from terminals 12, 13being connected across the whole of first sensing winding 14.

As another alternative, a separate coupling winding (not shown) may beprovided on core 9 and the output terminals 13, 15 or 12, 13 (as thecase may be) of amplifier 10 connected across the separate couplingwinding. The coupling winding to which the output of ampli fier 10 isconnected, may form part of a regenerative feedback circuit. This willbe clear to a man skilled in the art.

A non-regenerative circuit may also be used for coupling the output ofthe amplifier to the core, but with such an arrangement some form ofbias on the amplifier may be required.

A core balance earth leakage protective system according to theinvention is applicable to poly-phase power supply systems as well as tosingle phase power supply systems.

We claim 1. A core balance earth leakage protective system including amagnetic core; first and second sensing windings on the core; anelectronic amplifier, the input to the amplifier being connected tothefirst sensing winding on the core and the output from the amplifierbeing electro-magnetically coupled with the core; a polarized magneticrelay including an actuating coil which is connected to the secondsensing winding on the core, the relay being operative to causeisolation of load windings associated with the core when the currentthrough the relay coil exceeds a predetermined value.

2. A system as claimed in claim 1, wherein a plurality of load windingsare provided on the core.

3. A system as claimed in claim 1, wherein the core is arranged for aplurality of load conductors to be passed through or round the core toconstitute a plurality of load windings.

4. A system as claimed in claim 1, wherein the amplifier-part of thesystem is arranged to be more sensitive to earth faults than thepolarized relay part.

5. A system as claimed in claim 1, wherein the amplifier is of solidstate electronic type.

6. A system as claimed in claim 1, wherein the output from the amplifieris connected to a separate coupling winding on the core, therebyelectro-magnetically to couple the output with the core.

7. A system as claimed in claim 1, wherein the output from the amplifieris connected to at least part of the first sensing winding, therebyelectro-magnetically to

1. A core balance earth leakage protective system including a magneticcore; first and second sensing windings on the core; an electronicamplifier, the input to the amplifier being connected to the firstsensing winding on the core and the output from the amplifier beingelectro-magnetically coupled with the core; a polarized magnetic relayincluding an actuating coil which is connected to the second sensingwinding on the core, the relay being operative to cause isolation ofload windings associated with the core when the current through therelay coil exceeds a predetermined value.
 2. A system as claimed inclaim 1, wherein a plurality of load windings are provided on the core.3. A system as claimed in claim 1, wherein the core is arranged for aplurality of load conductors to be passed through or round the core toconstitute a plurality of load windings.
 4. A system as claimed in claim1, wherein the amplifier-part of the system is arranged to be moresensitive to earth faults than the polarized relay part.
 5. A system asclaimed in claim 1, wherein the amplifier is of solid state electronictype.
 6. A system as claimed in claim 1, wherein the output from theamplifier is connected to a separate coupling winding on the core,thereby electro-magnetically to couple the output with the core.
 7. Asystem as claimed in claim 1, wherein the output from the amplifier isconnected to at least part of the first sensing winding, therebyelectro-magnetically to couple the output with the core.
 8. A system asclaimed in claim 6, wherein the coupling winding forms part of aregenerative feedback circuit.
 9. A system as claimed in claim 7,wherein the output from the amplifier is connected to the first sensingwinding by impedance coupling.
 10. A system as claimed in claim 7,wherein the first sensing winding forms part of a regenerative feedbackcircuit.